Phthalate-based catalysts have been widely used in the commercial production of polypropylene worldwide. Extensive research and development have shown that the use of certain alkoxysilanes as external donors leads to optimum performance of phthalate-based catalysts (Kissin, Alkene Polymerizations with Transition Metal Catalysts, Studies in Surface Science and Catalysis 173: 224-243 (Elsevier, Amsterdam, 2007); Pasquini, Polypropylene Handbook (2nd Ed., Hanser Publishers, Munich, 2005); Moore, The Rebirth of Polypropylene: Supported Catalysts (Hanser Publishers, Munich, 1998)). The use of these silane donors allows for the production of a wide range of polypropylene products, due to the varied effect of these donors on the hydrogen response (characterized by melt flow rate (MFR)), isotacticity (characterized by % xylene solubles (% XS) or the isotactic pentad contents (mmmm) of the xylene-insoluble fraction (XI)), and molecular weight distribution (MWD).
Yet the use of these phthalate-based catalysts has caused regulatory and human health and safety concerns. Non-phthalate catalysts have been evaluated as replacements for the phthalate-based systems. However, the current non-phthalate catalysts generally do not match the performance of the phthalate systems.
Non-phthalate catalysts have certain limitations compared to the phthalate systems, for instance, very high initial activity, fast decay of polymerization activity, insufficient catalyst activity or low gas phase activity in sequential polymerization process for impact copolymer production. Additionally, non-phthalate catalysts often exhibit one or more of the following characteristics: low hydrogen response (characterized by low MFR) and low isotacticity. These limitations of the non-phthalate catalysts often render the properties of the polymers made with non-phthalate systems poorer than those of the polymers made with the phthalate-based catalysts.
There thus remains a need in the art to improve the technology of these non-phthalate catalyst systems to overcome the above-described limitations in the non-phthalate catalyst systems. This invention answers that need.